In recent years, severe dieback of boxwood (Buxus sempervirens) has been observed in the many forests in Turkey. In this study, boxwood diseases were investigated in the forests and nurseries in the Marmara and Black Sea regions of Turkey. Nurseries and boxwood forests in the above-mentioned regions were visited and leaf, branch, and soil samples were collected from symptomatic plants. Eleven fungal genera were isolated from the samples and identified based on morphological characteristics and sequencing of the ITS region and other barcoding loci. The box blight pathogen, Calonectria pseudonaviculata sensu stricto was most frequently isolated from declining boxwood both in the forest and nurseries. When tested on detached box shoots, C. pseudonaviculata was also the most aggressive pathogen on both wounded and intact leaves. Pseudonectria buxi (anamorph Volutella buxi) caused weak symptoms but only on wounded leaves. Pestalotiopsis asiatica and Gnomoniopsis smitovilgyi also caused necrosis on young shoots and yellowing on leaves. Phytophthora nicotianae was detected in one nursery while P. plurivora in two and P. occultans in one locations in the forests in the Black Sea region. The findings show that the boxwood decline in Black Sea region of Turkey is mainly caused by C. pseudonaviculata but other pathogens may also play a role in the decline. Commonly grown boxwood varieties; B. sempervirens ‘Osmanlı’, B. microphylla ‘Variegata’ and B. microphylla ‘Faulkner’ were found susceptible against box blight alone and mixed infections with Volutella blight while B. sempervirens ‘Rotundifolia’ was tolerant. These box varieties showed similar reactions against Phytophthora spp.